1. Field of the Invention
This invention relates to a method and apparatus for testing telecommunications equipment, and in particular telecommunications equipment over which two-way speech traffic is to be conveyed. The telecommunications equipment in question may be an existing telecommunications network, or part thereof, or individual items of telecommunications equipment not currently in service, for example prototypes of new designs of equipment.
2. Related Art
A number of methods for testing telecommunications equipment have been developed which monitor how the human auditory system responds to a test signal received over the equipment under test, in particular the extent to which distortions introduced by the network are perceptually significant to a listener. In particular the reader is referred to the present applicant""s earlier international patent applications, numbers W094100922, W095101011 and W095/15035. In these systems a test signal representative of human speech is transmitted over the system to be tested, and the distortions imposed on the signal by the equipment under test are analysed according to the relative perceptual importance of the distortion imposed. The perceptual distortion depends on such factors as the frequency response of the human auditory system, masking effects (e.g. quiet sounds being less noticeable if louder sounds occur shortly before or after them) and various other factors.
In practical usage most telecommunications systems used for speech carry two-way conversations, in which there are two parties speaking to each other. This affects the way the system responds to the signals input to it. For example, many bidirectional systems use voice activity detection to avoid excessive acoustic noise being transmitted when there is no speech on the respective channel. This results in temporal xe2x80x9cclippingxe2x80x9d, in which the beginning of a user""s utterance is lost because of the response time of the voice activity detector. The simulation of such characteristics would allow a system to be tested under more realistic conditions. However, the simulation of a conversation adds a number of complications to the measurement of signal quality.
Firstly, there are two separate signal paths (from the first user to the second user, and from the second user to the first user) and the signal quality will in general be different on the two lines, although such differences are likely to be relatively small in comparison with the differences between different point-to-point telecommunications links. It is therefore necessary to measure the signal quality of both paths.
Secondly, the perceived quality of each path has an effect on the other, because of the behaviour of the human users of the system. One aspect of human interaction is the natural changes of vocal level that occur between two parties during conversation. The levels at which an individual speaks depend on the level and quality of the speech received from the remote party. This means that during a telephone conversation, predominantly in the first few seconds, there is a subconscious adjustment in levels between users as a result of losses and distortions in the network path. This effect leads to a stable equilibrium for both parties. The users of a telephone line are therefore capable of adjusting their vocal output levels in accordance with the signal quality heard on the return path. Within certain limits, users adjust their vocal level according to this feedback. The level at which a user perceives he has to speak has an effect on the signal quality perceived by that user. The adjustment in vocal level itself also modifies the signal transmitted over the telecommunications system to the listener, and this in turn has an effect on the listening effort required of the other user. For example, if part of the telecommunications system is causing a large attenuation of the signal, then the listener will hear a very quiet received signal. Subconsciously assuming that this attenuation occurs in the return path as well, the listener will himself attempt to compensate by speaking louder. This affects certain characteristics of the voice itself, and the strong signal strength may also introduce distortions, for example xe2x80x98clippingxe2x80x99. These effects, and the greater signal strength itself, will be detected by the other user, who will in turn adjust his voice level accordingly.
Thus, the two-way nature of a typical telephone conversation allows users to compensate for some signal quality deficiencies, but this compensation may itself introduce further or different signal quality defects.
According to the present invention there is provided a method for testing telecommunications equipment over which two-way speech traffic can be carried between a first test point and a second test point, the method comprising the steps of;
a) generating first and second test signals representative of normal conversation, and each having a predetermined vocal level,
b) transmitting the first test signal from the first test point to the second test point,
c) simultaneously transmitting the second test signal from the second test point to the first test point,
d) measuring the quality of the test signals received at the respective test points,
e) determining a vocal level adjustment to be made in response to the quality measured at each test point, and
f) adjusting the vocal levels of the first and second test signals,
the said steps being repeated until a predetermined condition is reached, and an output being generated when said predetermined condition is reached, the output being indicative of the quality measured; wherein the quality measurement uses a perceptual analysis process which measures the extent to which the received signals carry distortions which would be perceptible to a human listener.
According to a further aspect of the invention there is provided apparatus for testing telecommunications equipment over which two-way speech traffic can be carried between a first test point and a second test point, the apparatus comprising;
means for generating first and second test signals representative of normal conversation, each having a predetermined vocal level;
means for supplying the first test signal to the first test point for transmission to the second test point;
means for detecting at the first test point, a signal received from the second test point;
means for measuring the quality of the detected signal by comparison with the second test signal, using a perceptual analysis process in which the extent is measured to which the received signals carry distortions which would be perceptible to a human listener;
means for determining a vocal level adjustment to be made in response to the measured quality;
means for adjusting the vocal level of the first test signal until a predetermined condition is reached; and
means for generating an output indicative of the quality measured when said predetermined condition is reached.
The invention therefore simulates the process by which the participants in a telephone conversation adjust their vocal levels during a telephone call until a predetermined condition, such as an equilibrium, is reached. The perceived signal quality at that equilibrium condition is regarded as the quality of the telecommunications connection which has been established for making the call. This equilibrium may be disturbed, and hence the quality changed, if the characteristics of the connection itself change, for example as a result of changes in radio link quality as a mobile telephone moves relative to its base station, or because of changes in ambient noise, but provided the external conditions remain unchanged the quality of the connection can be regarded as fixed for the duration of a call.
In a preferred arrangement the predetermined equilibrium condition referred to above is that the vocal level of the test signals (or at least one of them) is adjusted in response to the quality measured by less than a predetermined value, indicative that the iterative process is close to an equilibrium point. In practice, a human user continues to make further small adjustments throughout the telephone conversation, but these can be disregarded for the purposes of testing the equipment.
In general the telecommunications equipment under test will interact with the participants in the conversation by means of telephone handsets and other equipment interposed between the user and the network element under test, and these themselves introduce changes to the signal to be carried by the telecommunications equipment under investigation. Therefore, in a preferred embodiment there are interposed customer equipment interface simulators which simulate the modifications to a signal which occur between a talker and the telecommunications equipment under test, and between the telecommunications system under test and the listener. Typically such interfaces simulate the effects of the signal passing from the user""s mouth to that part of the telecommunications network which is under test, and from that part of the telecommunications equipment under test to the listener""s ear.
As well as the behaviour of the equipment itself, further factors which may be modelled by the customer equipment interface simulator are the effects of sound leakage in the telephone user/equipment interface. The term xe2x80x9cleakagexe2x80x9d in this context is used to mean any sound entering or leaving the system at the user/equipment interface. The effects to be simulated may include the loss of that part of the sound signal generated by the loudspeaker of a customer handset which is not received by the listener""s ear (outward earpiece leakage), the loss of that part of the sound signal uttered by a user which is not detected by the microphone in the telephone mouthpiece (outward mouthpiece leakage), any ambient background noise entering the microphone which does not emanate from the talker (inward mouthpiece leakage), and any ambient sound entering the ear which did not originate from the earpiece loudspeaker (inward earpiece leakage). The frequency response and magnitude of these effects will vary depending on the type of user/equipment interface to be modelled: for example a hands-free loudspeaker telephone suffers more leakage than an earpiece held to the ear, but the leakage at the earpiece varies more with frequency. These effects also depend on the actual acoustic characteristics of the earpiece and mouthpiece, such as their shape and acoustic impedance, and also on whether the earpiece(s) is designed for use with one or with both ears. Attenuation of the signals (to simulate signal loss), or superposition of ambient noise signals, may be applied to simulate these effects. The ambient noise signal is preferably modified to allow for any differences in the signal response of the microphone to the noise and to speech. Further factors characteristic of the customer equipment itself may also be used in determining the total vocal level.
In a typical telephone conversation the two users generally speak and listen alternately. However there are occasional periods of mutual silence in which neither participant is speaking, and there are also occasions when both users are speaking simultaneously. Each of the four situations can generate different signal quality measures at the two receiving ends.
A further factor which can affect perception of signal quality is the presence of noise. Acoustic noise is unwanted ambient sound detected by the mouthpiece of the user""s handset, or entering the user""s ear through earcap leakage. System noise is a spurious signal generated within the telecommunications system itself, and which generates an audible signal at the earpiece of a user handset. Noise may be present throughout the call, whether or not a speech-like signal is present, although in some circumstances low level noise may become perceptually important only when speech is absent, for example if gain control circuits amplify low-level noise in the absence of a strong signal. Some telecommunications equipment, in particular those of the xe2x80x98hands-freexe2x80x99 type, make use of voice activity detectors, such that no signal is transmitted unless a voice signal is present. In this case noise may only be audible when the speech-like signal is present. As already noted, temporal xe2x80x98clippingxe2x80x99 can occur with such devices. The test signal may model any one or more of these situations.
In one arrangement according to the invention, the test stimulus used comprises two signals for simultaneous transmission from respective test points, wherein the test stimulus comprises different segments in which;
a) only the first test point
b) only the second test point
c) both test points
d) neither test point;
are generating a signal having a spectral resemblance to human speech but not conveying intelligent content, and in which a noise signal is superimposed on one or both speech like signals.
In another arrangement a first measurement device at the first test point makes a call, through the system to be tested, to a second measurement device at the second test point, and the devices converse using predetermined speech signals, wherein the predetermined speech signals generated by each device are generated dynamically in response to the signals received by said devices, to simulate aspects of conversation over a non-perfect communications system link.
If a live system is to be tested, such that test signals are transmitted between two locations, it is convenient to use two test apparatus, one at each location, both test signals being generated at each location, one for transmission to the remote end, and the other for comparison with that received from the remote end. In laboratory conditions, in which an item of equipment is being tested in isolation, a single test apparatus may be connected to both ends of the equipment under test, and can analyse both paths.
The embodiment to be described models situations in which vocal level is quieter than the psycho-acoustically determined conversational equilibrium value. In some instances, where network mouth-to-ear sensitivity is very high, the vocal level at the ear may be too loud. Accordingly, in this embodiment of the invention, if the received vocal level exceeds a predetermined value, an output is generated to indicate an impaired signal quality.